Lithium iron oxide compounds have been extensively investigated during the last decade as alternative positive electrode materials to LiCoO2 and LiNiO2 having reduced cost and improved safety for lithium ion batteries.[1-3] For example, LiFeO2 has been prepared by ion exchange from α-NaFeO2, but it is metastable and leads to poor battery performance[4]. α-FeOOH (Akaganeite) is an iron oxy-hydroxide that has also been studied [5]. Although it has an attractive open structure with large tunnels, the lithiation process occurs at low voltage with poor rechargeability performance. From these two examples, several authors have related these results to an antagonist change of the bond structure of Fe4+/Fe3+ and Fe3+/Fe2+ redox couples within the O2− based oxide materials. Indeed, Fe4+/Fe3+ redox energy tends to lie too far below the Fermi level of a lithium anode. On the contrary, Fe3+/Fe2+ tends to lie too close to it, which lowers the voltage to a non-useful limit. To reduce the margin of this antagonist effect, Goodenough et al. introduced phosphorus within the iron oxide family in order to reduce the covalency of the Fe—O bond, which faces a strongly covalent P—O bond. Therefore, they investigated several phosphate materials like nasicon Li3Fe2(PO4)3 and olivine LiFePO4[2-3]. Since then, iron based phosphate materials are gaining much attention as positive active materials for consumer batteries.
Lithium extraction from LiFeIIPO4 (olivine) gives rise to FeIIIPO4 orthophosphate where the Fe2+/Fe3+ redox couple occurs at a constant voltage, 3.5V. The theoretical capacity is 170 mAh/g, with 160 mAh/g capacity available experimentally. Discharged and charged positive active materials, LiFePO4 and FePO4, respectively, have the same structural arrangement, i.e. same space group and close crystalline parameters, leading to a very good stability of the system during the electrochemical cycling process. This stability is not altered by Fe3+ ion generation, which is not the case when highly oxidizing Ni4+ ions are involved during the charge of LiMIIIO2 (M=Ni, Co) layered material. In addition, the cutoff voltage 3.7 V is not so high as to accelerate electrolyte degradation. LiFePO4 is an inexpensive material, nontoxic, and environmentally benign. For these reasons, olivine seems to be an attractive positive active material that could provide stable capacity and excellent calendar life.
Olivines, such as LiFePO4, are insulating materials, which seriously limits rate capability. Therefore, extensive work is in progress targeting the improvement of electronic conductivity by using carbon composite techniques, like carbon gel and sugar processes. So far, to get the desired conductivity, a conductor such as at least 15% of carbon additive has been needed to be mixed with the olivine active material. Unfortunately, the carbon is inactive, not contributing to the battery capacity. The challenge is to improve significantly the electronic conductivity and rate capability of the olivine using a reduced carbon ratio so as to minimize capacity loss and decreased energy density due to the inactive carbon. Like olivines, nasicons are insulating materials, with the same challenge to improve conductivity and rate capability using a reduced carbon ratio to minimize initial capacity loss and decreased energy density.